1. Field of the Invention
The present invention relates to an aluminum alloy casting having partially thick portions and being used for members having excellent toughness and high tensile strength together with requiring high strength such as aluminum wheels or other casting parts for a suspension system of an automobile.
2. Description of the Related Art
As far as an aluminum casting used as strength-requiring member for an automobile or the like is concerned, both strength and safety are important, in addition, excellent mechanical properties such as soundness, toughness, in particular high elongation, high 0.2% yield strength, and high tensile strength are required.
As to an alloy having such mechanical properties, there is an AC4CH (JIS) alloy of Al--Si--Mg alloy system. The AC4CH alloy usually comprises about 7% Si, 0.3% Mg and 0.2% (usually 0.15%) Fe, and have an excellent castability. In particular, when a low temperature of molten metal(s) or alloy(s) is casted into a mold, or at portions casted at high cooling rate of the temperature, for example, at thin skin portions, resulting castings will have rather good mechanical properties. As a result, the AC4CH alloy has been used for castings such as aluminum wheel, cross member and knuckle housing as a suspension apparatus part for an automobile, each of said members or parts require both strength and toughness.
Automobile parts, such as aluminum wheel, cross member, steering knuckles or the like, contain complicated figure of parts therein. Therefore, castability thereof is ensured by increasing the temperature of metal mold for casting at least 200.degree. C., preferably 300.degree. C. or more. Such a high metal mold temperature is liable to cause casting defects as well as to produce coarse crystallized substance in thick portions thereof wherein cooling rate of the temperature is rather slow. FIG.13 shows an example of crystallized substance comprising coarsed crystallized grains at the grain boundary, wherein the grain components are selected from the group consisting of at least two kinds of elements of Al, Si, Fe and Mg. In FIG.14(A) through FIG.14(D) there are shown enlarged scale of various forms of crystallized substances. FIG.14(A) shows an acicular crystallized substance (Al--Si--Fe alloy system), FIG.14(B) a skeleton-like crystallized substance (Al--Si--Fe alloy system), FIG.14(C) an Al--Si--Mg--Fe alloy system of crystallized substance, and FIG.14(D) a feather-like crystallized substance (Al--Si--Ti alloy system.). In the case such crystallized substances are formed there are easily occurred casting defects such as shrinkage holes, pin holes and the like as well as decreased mechanical properties especially in elongation.
As mentioned above, in a portion having a thick section, such as thick portion of an aluminum alloy casting, its cooling rate is slow so as to obtain inferior mechanical properties.
It is intended in the present invention to provide an aluminum alloy casting comprising the problems to be solved in the prior art as mentioned above, less decreased chemical properties and excellent toughness even under the condition of less cooling rate at the thick portion thereof as an automobile part such as aluminum wheel, suspension apparatus part or the like.